1. Field of the Invention
The present invention relates to a mobile communication system and a mobile terminal, and in particular, to a synchronizing method for synchronizing a spreading code in a reverse or upward communication line to be applied to a reservation code division multiple access (CDMA) packet communication system.
2. Description of the Related Art
A high-speed synchronizing method in a reverse line of a CDMA packet communication system has been described in pages 67 to 74 of an article entitled "A Demodulation for Direct-Sequence Spread ALOHA System", Technical Report of the Institute of Electronics, Information and Communication Engineers (IEICE), A P95-10 (1995-04).
FIG. 17 shows a conventional tranceiver section in a block diagram. In a mobile terminal, transmission data is multiplied by a spreading code generated from a PN generator 80. The obtained data is transmitted via a high-frequency circuit 404 from an antenna 400a. The spreading code generated from the PN generator 80 is equal to one symbol and such a code will be called "short code" herebelow. A receiver section of a base station receives a signal from the mobile terminal. The signal received by an antenna 400b is delivered via a high-frequency circuit 403 to be subjected to a correlation process in a matched filter 601. To extract received data from a signal outputted from the matched filter, an initial acquisition circuit 603 detects a preamble 202 of a packet, which will be described later, and then produces a synchronizing signal. Using the synchronizing signal produced from the initial acquisition circuit 603 as a reference signal, a packet de-multiplexer 605 samples the signal from the filter 601 at a symbol period or for each symbol to produce an associated value. The signal outputted from the de-multiplexer 605 is demodulated by a detector circuit 607.
FIG. 2 shows a format of packets communicated in the conventional example. The packet includes, as shown in (A) of FIG. 2, a preamble 202a of which each bit is "1", a frame start delimiter 203a to separate the preamble from data, and information data 204a in this order. As can be seen from (B) and (C), a short code equivalent to the symbol period is employed as the spreading code.
As described above, in the CDMA packet communication system proposed according to the prior art, a short code is adopted as the spreading code because of restriction of time required for acquisition. However, when the short code is used in the CDMA communication system, interference occurs between the spreading codes and hence the number of communicable terminals is decreased. Although it is desired to employ a long code (having a period of several symbols), there arises a problem that the long code requires a longer period of time for acquisition. To reduce the time required for acquisition, it is necessary to utilize a matched filter conducting a larger number of operations, which is not practical because of limitation of the circuit size.
Reference may further be made to U.S. patent application Ser. No. 08/690,819 filed on Aug. 1, 1996 and entitled "CDMA MOBILE COMMUNICATION SYSTEM & COMMUNICATION METHOD".